StreamingUpdateSolrServer buffers all added documents and writes them
into open HTTP connections. This class is thread safe.
Although any SolrServer request can be made with this implementation,
it is only recommended to use StreamingUpdateSolrServer with
/update requests. The base class {&link CommonsHttpSolrServer} is
better suited for the query interface, or for situations where you
need to detect errors or catch exceptions from udpate requests.